1. Field of the Invention
This invention relates to inkjet printers and, in particular, to a printing technique for minimizing unequal swath boundary behavior across print regions produced by an inkjet printer.
2. Background
This application is related to U.S. patent application Ser. No. 09/329,974, entitled xe2x80x9cDYNAMIC ADJUSTMENT OF UNDER AND OVERPRINTING LEVELS IN A PRINTERxe2x80x9d, filed on Jun. 10, 1999, now U.S. Pat. No. 6,132,021 and assigned to the common assignee. This prior application is incorporated herein by reference. The prior application provides a dynamic adjustment of the fluid volume used for underprinting and/or overprinting pigment-based inks (or other inks) for speeding up the drying time of the pigment-based ink or improving its adherence to a medium. This technique works well, however, the present Applicants have discovered that white space swath boundary banding occurs in printed areas, which contain underprinted black ink, especially in printing a black object whose height is greater than the printer""s swath height
A typical high quality color inkjet printer prints using at least four colors of ink: cyan, magenta, yellow, and black. A common black ink is a pigment-based ink where undissolved particles are suspended in a clear vehicle. Such pigment-based ink creates the darkest black with a minimum of bleed into the paper. Since the paper is typically white, any significant bleeding of the black ink into the paper will noticeably reduce the sharpness of the edges of black text or other black graphics.
For non-black color inks, dye-based inks are very popular. Dye-based inks do not have color particles suspended in solution and thus tend to bleed into the paper more than pigment-based inks. Since the dye-based ink wicks or bleeds into the paper, the dye-based inks dry faster than the pigment-based inks, which effectively pool on the paper surface. Non-black color inks may also be pigment-based.
Examples of such inks are described in U.S. Pat. Nos. 5,695,820 and 5,626,655 assigned to the present assignee and incorporated herein by reference.
The prior application provides dynamic adjustment of the fluid volume used for underprinting and/or overprinting pigment-based inks (or other inks) for speeding up the drying time of the pigment-based ink or improving its adherence to a medium. The invention identifies a characteristic in the printer that affects the optimum volume of under/overprinted fluid to be printed, such as pen temperature, pen operating frequency, pen operating life, ambient temperature, and ambient humidity, and varies the under/overprinted fluid accordingly.
However solid black with underprinted cyan and magenta can have white space swath boundary banding. In printing systems using underprinting, solid black, cyan and magenta fills are band-free, however the combination of these three colors in an underprinted black area exhibits swath boundary banding. One reason for the banding is that black pigments migrate away from swath boundaries of under printed black ink. FIG. 1A shows an example of white space swath boundary banding at underprinted black ink. Cyan and magenta are underprinted under solid black ink. The white space boundary occurs if the printed object is bigger than the print head height (in this illustration, the height is xc2xdxe2x80x3). The white space boundary in FIG. 1A is between Swath 1 and Swath 2 for an object that is solid black and greater than xc2xdxe2x80x3.
Therefore, what is needed is a technique that can reduce white space swath boundary banding while providing an optimum amount of underprinting fluid as characteristics within the printer change.
The present invention provides a system to underprint a black object whose height is greater than one swath height of an inkjet printer. In the following examples, it will be assumed that the black ink is pigment-based, and it is desired to underprint the black ink with either a dye-based color ink or a fixer.
A method and apparatus in an inkjet printer for printing a black object whose height is greater than the swath height of the inkjet printer is provided. The printer includes one or more printheads that moves relative to a medium while the printheads are printing on the medium. The printheads include a first set of nozzles for printing dots of a first ink, and a second set of nozzles for printing dots of a second fluid. The process adjusts the volume of the first ink, wherein the first ink is black ink, which is printed over the second fluid. The black ink volume is greater than the volume of the second fluid. The volume of black! ink is increased by using a greater number of nozzles in the first set compared to the number of nozzles used in the second set.
One of the advantages of the present invention is that white space boundary banding is diminished by increasing the number of nozzles for printing a black object whose height is greater than one swath height.